The backing plates for my keel bolts are galvanised mild steel, the galvanising is long gone and they are now heavily corroded: they had been covered with a thin layer of fibreglass, it cracked and they got wet.
I’ve previously tried knocking the rust off and coating with rust cure and epoxy, it looked ok for a short time but 4 years down the line the problem is back and a proper job is being considered. A proper job would seem to be replacing them with stainless steel. However this looks like a difficult exercise offering many opportunities to fuck up.
Aside from the usual issues around removing the nuts (they may be galled, they don’t respond to my efforts with a spanner, I’m wary of pulling the bolts out/breaking them etc) and how much to torque them if I ever get to the point of putting them back on again there are problems created by the sheer size of the backing plates. The plates extend either side of the keel area into the bilges, i.e. they have a bit of a curve to them. Most of them require two perfectly positioned holes so that the new plate can be dropped over the bolts (hopefully that won’t involve cutting chunks out of the cabin sole). The bolts are M24. The boat is out of the water with the mast up, I’d imagine replacing one plate at a time.
I gather that later models of the same boat (mine is one of the first) had the more common single-square-backing-plate per bolt. However I haven’t yet been able to find out whether there were other changes that made that a sensible arrangement. There are around 1900 Kg of keel bolted straight through around an inch of glass fibre hull, so I suppose a certain amount of load distribution is required to keep the 2 parts attached. It has hung on for 40 years. The builders went bust a long time ago, the designer passed away a few years ago.
Photos documenting the work done previously are here: https://photos.app.goo.gl/T1fZ5E1oXXYEjxNu2.
As the forum which seems to have people who know what they are talking about I come here to ask: What to do? How to do it?
I’ve previously tried knocking the rust off and coating with rust cure and epoxy, it looked ok for a short time but 4 years down the line the problem is back and a proper job is being considered. A proper job would seem to be replacing them with stainless steel. However this looks like a difficult exercise offering many opportunities to fuck up.
Aside from the usual issues around removing the nuts (they may be galled, they don’t respond to my efforts with a spanner, I’m wary of pulling the bolts out/breaking them etc) and how much to torque them if I ever get to the point of putting them back on again there are problems created by the sheer size of the backing plates. The plates extend either side of the keel area into the bilges, i.e. they have a bit of a curve to them. Most of them require two perfectly positioned holes so that the new plate can be dropped over the bolts (hopefully that won’t involve cutting chunks out of the cabin sole). The bolts are M24. The boat is out of the water with the mast up, I’d imagine replacing one plate at a time.
I gather that later models of the same boat (mine is one of the first) had the more common single-square-backing-plate per bolt. However I haven’t yet been able to find out whether there were other changes that made that a sensible arrangement. There are around 1900 Kg of keel bolted straight through around an inch of glass fibre hull, so I suppose a certain amount of load distribution is required to keep the 2 parts attached. It has hung on for 40 years. The builders went bust a long time ago, the designer passed away a few years ago.
Photos documenting the work done previously are here: https://photos.app.goo.gl/T1fZ5E1oXXYEjxNu2.
As the forum which seems to have people who know what they are talking about I come here to ask: What to do? How to do it?
). From the pictures there isn't really much curvature. I'd use flat plates and bed them in thickened epoxy to handle any curve. You are going to want to do that anyway to ensure good contact. 24mm holes are a little tough to drill in a good material unless you have a heavy drill press, a milling machine even better. You may have more luck buying an 
